A day-centered approach to modeling cortisol: diurnal cortisol profiles and their associations among U.S. adults.

Diurnal cortisol is a marker of HPA-axis activity that may be one of the biological mechanisms linking stressors to age-related health declines. The current study identified day-centered profiles of diurnal cortisol among 1101 adults living in the United States. Participants took part in up to four consecutive days of salivary cortisol collection, assessed at waking, 30min post-waking, before lunch, and before bedtime. Growth mixture modeling with latent time basis was used to estimate common within-day trajectories of diurnal cortisol among 2894 cortisol days. The 3-class solution provided the best model fit, showing that the majority of study days (73%) were characterized by a Normative cortisol pattern, with a robust cortisol awakening response (CAR), a steep negative diurnal slope, coupled with low awakening and bedtime levels. Relative to this profile, diurnal cortisol on the remainder of days appeared either elevated throughout the day (20% of days) or flattened (7% of days). Relative to the normative trajectory, the elevated trajectory was distinguished by a higher morning cortisol level, whereas the flattened trajectory was characterized by a high bedtime level, with weaker CAR and diurnal slope parameters. Relative to the normative profile, elevated profile membership was associated with older age and cigarette smoking. Greater likelihood of the flattened cortisol pattern was observed among participants who were older, male, smoked cigarettes, used medications that are known to affect cortisol output, and reported poorer health. The current study demonstrates the value of a day-centered growth mixture modeling approach to the study of diurnal cortisol, showing that deviations from the classic robust rhythm of diurnal cortisol are associated with older age, male sex, use of medications previously shown to affect cortisol levels, poorer health behaviors, and poorer self-reported health.